Dr Martin HendryUniversity of Glasgow

Dark Energy

Cold Dark Matter

Ato

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State of the Universe – Nov 2003

The Runaway The Runaway UniverseUniverseDr Martin HendryDept of Physics and AstronomyUniversity of Glasgow

Cosmology – the study of the Universe as a whole:

• Origin

• Evolution

• Eventual Fate

Galileo Galilei:1564 – 1642 AD

Galileo Galilei:1564 – 1642 AD

“I have observed the nature and the material of the Milky Way. With the aid of the telescope this has been scrutinized so directly and with such ocular certainty that all the disputes which have vexed philosophers through so many ages have been resolved, and we are at last freed from wordy debates about it.

The galaxy is, in fact, nothing but a collection of innumerable stars grouped together in clusters. Upon whatever part of it the telescope is directed, a vast crowd of stars is immediately presented to view. Many of them are rather large and quite bright, while the number of smaller ones is quite beyond calculation.”

from The Starry Messenger (1610)

The stars are VERY far away. The nearest star (after the Sun) is about 40 million million km from the Earth. It takes light more than 4 years to cross this distance (travelling at a speed of 300,000 km per second)

If the Earth-Sun distance were the width of this screen, the nearest star would be in Paris !!!

Measuring Astronomical Distances: Parallax

Earth in July

Earth in JanuarySun

Nearby star

Measuring Astronomical Distances: Parallax

Earth in July

Earth in JanuarySun

Nearby star

View from the Earth in January

Measuring Astronomical Distances: Parallax

Earth in July

Earth in JanuarySun

Nearby star

View from the Earth in January

View from the Earth in July

Measuring Astronomical Distances: Parallax

View from the Earth in January

View from the Earth in July

Even the nearest star shows a parallax shift of only 1/2000th the width of the full Moon

Cepheid Variables: Cosmic Yardsticks

Henrietta Leavitt1908-1912

The nature of the nebulae?…

Early 20th Century

Gas clouds within the Milky Way, or Island Universes?….

The Great Debate, 1920

Shapley vs Curtisat the National Academy of Sciences

The Great Debate, 1920

Shapley vs Curtis

Shapley argues successfully that the nebulae are within the Milky Way

at the National Academy of Sciences

1922: Hubble finds Cepheids in the Great Nebula in Andromeda

Hubble measured distances to dozens of nearby nebulae

Even the nearest, in Andromeda, was

millions of light years distant

Hubble also measured the shift in colour, or wavelength, of the light from distant galaxies.

Galaxy

Hubble also measured the shift in colour, or wavelength, of the light from distant galaxies.

Galaxy

Laboratory

Hubble also measured the shift in colour, or wavelength, of the light from distant galaxies.

Galaxy

Laboratory

Hubble’s Law: 1922Hubble’s Law: 1922

Distant galaxies are receding from us with a velocity proportional to their distance

‘Recession of the Nebulae’ caused not by the motion of galaxies through space, but the expansion of space itself between the galaxies

Hubble’s Interpretation

How fast is the Universe expanding?How fast is the Universe expanding?

Principal difficulty has been local distortions in ‘Hubble flow’

e.g. spectrum of M31 is blueshifted

Galaxies are clustered

Structure in the Universe assembled by gravity

Galaxies are clustered

Structure in the Universe assembled by gravity

Locally, gravity sufficient to overcome cosmic expansion

Galaxies are clustered

Structure in the Universe assembled by gravity

Locally, gravity sufficient to overcome cosmic expansion

On larger scales, expansion diluted: galaxies have peculiar velocity on top of Hubble velocity

Main local distortion due to Virgo cluster

Problem:

Need to determine H0 from remote galaxies, where peculiar motions are less important….

….but….

We cannot use primary distance indicators to measure their distance

Need Distance Ladder!!

HST has ‘bypassed’ one stage of the Distance Ladder, by observing Cepheids beyond the Local Group of galaxies

This has dramatically improved measurements of H0

Virgo Cluster galaxyM100, 60 million light years distant…..

HST Key Project, led by Wendy FreedmanMeasure Cepheid distances to ~30 nearby galaxies,

Link Cepheids to Secondary distance indicators

Must ensure that remote galaxy data are free from Selection Effects

e.g. intrinsically brighter or bigger?…

Must ensure that remote galaxy data are free from Selection Effects

e.g. intrinsically brighter or bigger?…

Malmquist Bias

Must ensure that remote galaxy data are free from Selection Effects

e.g. intrinsically brighter or bigger?…

Malmquist Bias

Will the Universe continue Will the Universe continue to expand forever?to expand forever?

To find out we need to compare the expansion rate now with the expansion rate in the distant past…

Is the Universe speeding up or slowing down?

Answer depends on the geometry of the Universe

Answer depends on the geometry of the Universe

Closed

Answer depends on the geometry of the Universe

Closed Open

Answer depends on the geometry of the Universe

Closed Open Flat

We can measure this using Supernovae and the background radiation

Geometry of the Universe affects the relationship between distance and redshift of the supernovae

Closed Open Flat

We can measure this using Supernovae and the background radiation

Early Universe too hot for neutral atoms

Free electrons scattered light (as in a fog)

After 300,000 years, cool enough for atoms; fog clears!

Background radiation predicted in 1950s and 1960s by Gamov, Dicke, Peebles.

Discovered in 1965 by Penzias and Wilson

Arno Penzias and Robert Wilson

Robert Dicke

Jim Peebles

Cosmic Background Explorer (CoBE), launched 1989

CoBE map of temperature across the sky

CoBE map of temperature across the sky

CMBR ‘ripples’ are the seeds of today’s galaxies

Galaxy formation is highly sensitive to the pattern, or power spectrum, of CMBR temperature ripples

Position of first peak sensitive probe of thegeometry of the Universe

Hot off the press!…

Microwave Anisotropy Probe

First year WMAP results published Tuesday 11th Feb

First year WMAP results published Tuesday 11th Feb

From Bennett et al (2003)

Position of first peak sensitive probe of thegeometry of the Universe

Answer depends on the geometry of the Universe

Closed Open Flat

Results:

The expansion is accelerating

The geometry of the Universe is FLAT

The Universe will continue to expand indefinitely

What is driving the cosmic acceleration?…What is driving the cosmic acceleration?…